Definition of atomic mass and molecular mass

Atomic Mass

It is known that the atomic mass of an element/compound is not the mass of one atom or molecule of the said element/compound because they occur as a mixture of isotopes. As the atomic and molecular masses are expressed on a relative scale based on a mass of 12C atom, all atomic and molecular masses are in fact the weighted average of the mass of these isotopes. Thus, atomic mass of an element is defined as the average relative mass of an atom of an element as compared to the mass of an atom of carbon (12C ) taken as 12.

definition of atomic mass

For example, chlorine occurs in nature in the form of two isotopes with atomic mass 35 and 37 in the ratio of 3:1. Therefore,

average atomic mass of chlorine

Thus, an atom of chlorine is 35.5 times heavier that 1/12th of the mass of a 12C atom.

Gram-Atomic-Mass">

Gram Atomic Mass

The quantity of an element whose mass in grams is numerically equal to its atomic mass. In simple terms, atomic mass of an element expressed in grams is the gram atomic mass or gram atom.

For example, the atomic mass of oxygen = 16 amu

Therefore gram atomic mass of oxygen = 16 g

The gram atom should not be confused with the mass of one atom of the element in grams, which is the actual mass of the atom.Molecular-Mass">

Molecular Mass

Molecular mass of a substance is defined as the average relative mass of its molecule as compared to the mass of an atom of 12C taken as 12. It expresses as to how many times the molecule of a substance is heavier than 1/12th of the mass of an atom of carbon.

For example, a molecule of carbon dioxide is 44 times heavier than 1/12th of the mass of an atom of carbon. Therefore the molecular mass of CO2 is 44 amu.Gram-Molecular-Mass">

Gram Molecular Mass

A quantity of substance whose mass in grams is numerically equal to its molecular mass is called gram molecular mass. In simple terms, molecular mass of a substance expressed in grams is called gram molecular mass.

For e.g., the molecular mass of oxygen = 32 amu

Therefore, gram molecular mass of oxygen = 32 g

The gram molecular mass should not be confused with the mass of one molecule of the substance in grams, which is the actual mass of the molecule.

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